Transistors, whether of the bipolar or field-effect types, are frequently used as switches to supply a load with switching currents which may be quite large. Especially where the transistor load is an inductive one, the switch breaking, that is the sudden transition from the conduction state to the cutoff state of the transistor, brings about a transient situation of overvoltage across the power terminals of the transistor, that is between its collector and emitter in the instance of a bipolar transistor or between its drain and source terminals in the instance of a field-effect transistor. This overvoltage may be several times higher than the transistor supply voltage and attain such values and durations as to exceed safe limits set by the structural characteristics of the transistor, to the point that the transistor may be destroyed.
A known practice for protecting a bipolar transistor in such circumstances is to connect, between the collector and the base of the transistor, a device with a predetermined unidirectional conduction threshold, such as a Zener diode having a lower reverse breakdown voltage than the breakdown voltage between the transistor collector and emitter, with the base open or floating, referred to as BV.sub.ceo. As the collector/emitter voltage increases due to the transition from the conduction to the cutoff state up to the reverse conduction value of the Zener, a current is injected into the transistor base. The latter is, therefore, restored to conduction and discharges the energy stored in the inductive load on itself. Of course, the transistor should be sized to absorb such energy harmlessly. The maximum overvoltage between the collector and the emitter would equal here the breakdown or reverse conduction threshold voltage of the Zener diode plus the transistor base/emitter/forward voltage.
A similar device to the above-described one is used to protect a field-effect power transistor, such as a vertical MOSFET, against overvoltages across the source and drain terminals. But in this case the structure is somewhat different, because a diode with opposite polarity from that of the Zener diode must be connected in series with the Zener diode connected between the gate and drain terminals. In fact, the gate terminal voltage may exceed by some 10-15 volts the drain terminal voltage in normal operation of the transistor, so that a Zener diode connected directly between the gate and the drain would disallow such operating conditions because it would be in forward conduction the very moment that the gate/drain voltage exceeds the forward conduction threshold (typically 0.7 volts at the silicon) of the Zener diode.
It is recognized that, to provide a protection device of the aforesaid kind, one may use, for the predetermined conduction threshold component, an open-base bipolar transistor connected between the emitter and the collector by utilizing the base punch-through effect, or use a diode which, while not being operated on Zener's effect, has a conduction characteristic similar to that of a Zener diode, i.e. a non-destructive reverse breakdown voltage having a predetermined threshold, like most of the diodes including an avalanche reverse breakdown junction.
If necessary, instead of a single Zener diode or its equivalent, there may be connected in series several, until the desired threshold voltage for protection is attained. The need for this may arise where the protection device is to be implemented in a monolithic integrated structure within a well-defined manufacturing process that ideally should not be altered by the addition of specific steps directed to form a diode with the reverse breakdown characteristics sought; that is, it is preferred to connect in series several junctions which can be easily obtained by the standard process. In this case, however, the serial resistance of the protection device in conduction may be comparatively high, which is apt to impair the protection effectiveness, especially where the transistor to be protected is a bipolar one.
To obviate this problem, it has been known to use an additional bipolar transistor serving as a current amplifier. That transistor, as shown at T1 in FIG. 1 of the accompanying drawings, has its collector in common with the collector of the transistor to be protected; the latter being in this example the power transistor T2 of an NPN Darlington pair whose drive transistor is designated T3. Further, the transistor T1 has its emitter connected to the base of the power transistor T2 and its base connected to the anode of the first Zener diode Zn in a chain of n Zener diodes Z1, Z2 . . . , Zn connected, with the other end, to the common collectors of the transistors.
An integrated protection device useful especially with a MOSFET transistor is disclosed in German Patent Application DE 4,122,347. The circuit diagram of that device is shown in FIG. 2 of the accompanying drawings applied to an N-channel MOSFET transistor indicated at T4. The component with Zener diode function, i.e. with predetermined reverse breakdown unidirectional conduction, is represented there by a series of n Zener diodes Z1, Z2 . . . Zn to indicate that it is formed from n junctions connected serially together. Thus, an overall breakdown or reverse conduction voltage is obtained which is a multiple of that to be obtained with a single junction formed by the particular manufacturing process adopted. As may be seen, provided in series with the diodes Z1-Zn is a diode D1 connected in reverse polarity with respect to the Zener diodes in order to inhibit forward conduction between the gate G and the drain D. However, that prior structure cannot provide accurate and repeatable reverse conduction threshold voltages because the junctions of the component diodes are formed between diffused regions, and accordingly, do not have reverse breakdown voltages that can be accurately predicted. In addition, its dimensioning is fairly critical due to the risk of parasitic effects between components of the device, and is unsuited to applications where the protection device is integrated along with control circuits for the power transistor to be protected.